Lithographic apparatus, cleaning system and cleaning method for in situ removing contamination from a component in a lithographic apparatus

ABSTRACT

An immersion lithographic apparatus includes a cleaning system for cleaning a component in the immersion lithographic apparatus in situ. The cleaning system is arranged to provide a cleaning environment in proximity of a predetermined position on a component to be cleaned. The system is also arranged to provide the cleaning environment substantially independent of a type of contamination present at the predetermined position.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/957,754, filed on Oct. 5, 2004 now U.S. Pat. No. 7,385,670, thecontent of which is incorporated herein by reference in its entirety.

FIELD

The present invention relates to an immersion lithographic apparatus, acleaning system, and a cleaning method for in situ removal ofcontamination from a component in an immersion lithographic apparatus.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

Although lithographic apparatus are operated in clean rooms and flushedwith clean air, contamination of the apparatus does occur and, dependingon the location and type of contaminant, causes various problems. Forexample, inorganic contaminants on the mask deriving from the air in theclean room or from manufacture, transportation and storage of the maskcan cause localized absorption of the projection beam leading to doseerrors and improper imaging of mask features or even printing of marksin what should be blank areas. Particulates on the substrate table candistort the substrate leading to localized focus errors (known as hotspots). When due to contamination on a substrate table, the substrateadopts a bended position, and the pattern on the surface of the bendedsubstrate does not correspond to the pattern intended to be transferredto the substrate, thereby contributing to bad overlay.

In addition to the ambient air and the manufacture, etc., of masks andsubstrates, sources of contamination include resist debris sputteredfrom the substrate by the projection beam during exposures, andmechanical contact between moving parts of the apparatus, which maycause particulates to be dislodged from the contacting surfaces.Contamination may also include metal and/or oxide particles.

Most of the contamination is presumably entering the lithographicapparatus via the substrates which have been treated in processes priorto entering the apparatus. In particular, resist contamination isthought to enter the apparatus via the wafers.

To minimize errors caused by contamination, susceptible parts of theapparatus, such as masks, mask tables, and substrate tables, are cleanedfrequently. This generally is a time-consuming manual task, taking twohours or more to clean a substrate table, for example, which causesundesirable downtime of the apparatus and must be carried out by skilledengineers. On occasion, manual cleaning fails to remove the contaminantsand must be repeated. Selective cleaning of a burl table is disclosed inEP-1 093 022-A in which an abrasive tool or electromagnetic radiation ofunspecified form is used. U.S. Pat. No. 6,249,932 discloses a manualcleaning head that uses blown air and vacuum for cleaning a table in alithographic projection apparatus. Various methods of cleaningsubstrates are known—see e.g. WO 02/053300 and WO 02/42013—but theserequire the substrates to be placed in special machines.

To overcome the problems of downtime, the lithographic apparatus may beprovided with a cleaning device for cleaning in situ a component in thelithographic apparatus, as disclosed in EP 1329773 A2. The cleaningdevice may provide a laser to a blade and/or thermally dislodgecontaminants. The wavelength of the laser beam is chosen such thatabsorption by the contaminants that are expected to be present on thecomponent be cleaned, is maximal. When a short pulse length is used, inthe order of less than 100 nanoseconds, the sudden thermal expansiondifference between the heated contamination and the component may causea shockwave, resulting in G-forces high enough to loosen thecontamination from the component. A clean device may additionally, oralternatively, provide in a non-ionizing, low pressure environmentaround the component to be cleaned, so that electrostatic forces due toa potential difference between a cleaning tool and the component to becleaned are created. The potential difference and separation between thetool and the component to be cleaned that are needed to removecontaminants depends on the contaminants to be removed and theproperties of the surface to which they are adhered. In other words, ineither way, successfully operating the cleaning device depends on priorknowledge about the type of contamination to be removed.

SUMMARY

It is an aspect of the present invention to provide an immersionlithographic apparatus with a more versatile cleaning facility.

It is an aspect of the present invention to provide a versatile cleaningsystem for in situ cleaning of a component in an immersion lithographicapparatus.

It is an aspect of the present invention to provide a versatile cleaningmethod for in situ removal of contamination from a component in animmersion lithographic apparatus.

According to an aspect of the invention, there is provided an immersionlithographic apparatus that is arranged to transfer a pattern from apatterning device onto a substrate. The apparatus includes a cleaningsystem for cleaning a component in the immersion lithographic apparatusin situ. The cleaning system is arranged to provide a cleaningenvironment in proximity of a predetermined position on a component tobe cleaned. The system is further arranged to provide this cleaningenvironment substantially independent of a type of contamination presentat the predetermined position.

According to an aspect of the invention, there is provided an immersionlithographic apparatus. The apparatus includes a substrate tableconstructed and arranged to support a substrate, a projection systemconstructed and arranged to project a patterned beam of radiation onto atarget portion of the substrate, and a liquid that fills a space betweenthe projection system and the substrate. The apparatus includes acleaning system for cleaning a component in the immersion lithographicapparatus in situ, said cleaning system being arranged to provide acleaning environment in proximity of a predetermined position on thecomponent to be cleaned. The system is further arranged to provide thecleaning environment substantially independent of a type ofcontamination present at the predetermined position.

According to an aspect of the invention, there is provided a cleaningsystem for cleaning a component in an immersion lithographic apparatusin situ. The immersion lithographic apparatus includes a substrate tableconstructed and arranged to support a substrate, a projection systemconstructed and arranged to project a patterned beam of radiation onto atarget portion of the substrate, and a liquid that fills a space betweenthe projection system and the substrate. The cleaning system is arrangedto provide a cleaning environment in proximity of a predeterminedposition on the component that is substantially independent of a type ofcontamination.

According to an aspect of the invention, there is provided a cleaningmethod for removing contamination from a component in an immersionlithographic apparatus in situ. The immersion lithographic apparatusincludes a substrate table constructed and arranged to support asubstrate, a projection system constructed and arranged to project apatterned beam of radiation onto a target portion of the substrate, anda liquid that fills a space between the projection system and thesubstrate. The method includes creating a cleaning environment inproximity of a predetermined position on the component and that isindependent of a type of contamination to be removed, and removing thecontamination from the component in situ.

According to an aspect of the invention, there is provided a cleaningmethod for removing contamination from a component in an immersionlithographic apparatus in situ. The immersion lithographic apparatusincludes a substrate table constructed and arranged to support asubstrate, a projection system constructed and arranged to project apatterned beam of radiation onto a target portion of the substrate, anda liquid that fills a space between the projection system and thesubstrate. The method includes identifying a predetermined position tobe cleaned on the component, and cleaning the predetermined positionwith a cleaning system in situ, the cleaning system being arranged toprovide a cleaning environment in proximity of the predeterminedposition that is substantially independent of a type of contamination.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts schematically a part of an embodiment of the invention;

FIG. 3 depicts schematically a part of an embodiment of the invention;

FIG. 4 depicts schematically a part of an embodiment of the invention;

FIG. 5 depicts schematically a part of an embodiment of the invention;and

FIG. 6 depicts schematically a part of an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus may include, as shown, anillumination system (illuminator) IL configured to condition a radiationbeam B (e.g. UV radiation); a support structure (e.g. a mask table) MTconstructed to support a patterning device (e.g. a mask) MA andconnected to a first positioner PM configured to accurately position thepatterning device in accordance with certain parameters; a substratetable (e.g. a wafer table) WT constructed to hold a substrate (e.g. aresist-coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate in accordance withcertain parameters; and a projection system (e.g. a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g. including one or more dies) of the substrate W.

The apparatus includes a cleaning system CS for cleaning a component inthe lithographic apparatus in situ, thereby avoiding the need to open upand dismantle the apparatus to remove the component to be cleaned. Thecleaning system CS is arranged to provide a cleaning environment in atleast the proximity of a predetermined position on the component to becleaned. The cleaning system CS is further arranged to provide thiscleaning environment substantially independent of a type ofcontamination present at the predetermined position. The open arrowsshown in FIG. 1 indicate possible predetermined positions and theirproximities.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic,electrostatic, or other types of optical components, or any combinationthereof, for directing, shaping, and/or controlling radiation.

The support structure MT supports, i.e. bears the weight of, thepatterning device MA. It holds the patterning device MA in a manner thatdepends on the orientation of the patterning device MA, the design ofthe lithographic apparatus, and other conditions, such as whether or notthe patterning device MA is held in a vacuum environment. The supportstructure MT may use mechanical, vacuum, electrostatic or other clampingtechniques to hold the patterning device MA. The support structure MTmay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure MT may ensure that the patterning deviceMA is at a desired position, for example, with respect to the projectionsystem PS. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” as used herein should be broadlyinterpreted as referring to any device that can be used to impart aradiation beam with a pattern in its cross-section such as to create apattern in a target portion of the substrate. It should be noted thatthe pattern imparted to the radiation beam may not exactly correspond tothe desired pattern in the target portion of the substrate, for example,if the pattern includes phase-shifting features or so-called assistfeatures. Generally, the pattern imparted to the radiation beam willcorrespond to a particular functional layer in a device being created inthe target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” as used herein should be broadlyinterpreted as encompassing any type of projection system, includingrefractive, reflective, catadioptric, magnetic, electromagnetic andelectrostatic optical systems, or any combination thereof, asappropriate, for the exposure radiation being used, or for otherfactors, such as the use of an immersion liquid or the use of a vacuum.Any use of the term “projection lens” herein may be considered assynonymous with the more general term “projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). Alternatively, the apparatus may be of atransmissive type (e.g. employing a transmissive mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery systemincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases, the source may be an integral part of thelithographic apparatus, for example, when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator and a condenser.The illuminator may be used to condition the radiation beam, to have adesired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF2 (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor IF1 can be used to accurately position themask MA with respect to the path of the radiation beam B, e.g. aftermechanical retrieval from a mask library, or during a scan. In general,movement of the mask table MT may be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioner PM. Similarly,movement of the substrate table WT may be realized using a long-strokemodule and a short-stroke module, which form part of the secondpositioner PW. In the case of a stepper (as opposed to a scanner) themask table MT may be connected to a short-stroke actuator only, or maybe fixed. Mask MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks as illustrated occupy dedicated targetportions, they may be located in spaces between target portions (theseare known as scribe-lane alignment marks). Similarly, in situations inwhich more than one die is provided on the mask MA, the mask alignmentmarks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIGS. 2-6 each schematically show a part of an embodiment of theinvention. FIG. 2-4 show schematically a cleaning system for in situcleaning a component in a lithographic apparatus. By way of example, thecomponent to be cleaned is the substrate table WT. It should, however,be clear that the invention may be applied with the aim to clean othercomponents. The cleaning system CS is arranged to provide a cleaningenvironment in at least a proximity of a predetermined position PP onthe component to be cleaned. The cleaning environment is an environmentthat is effective in removing contamination from the component to becleaned.

The system is further arranged to provide this cleaning environmentsubstantially independent of a type of contamination present at apredetermined position. An advantage is that no specific knowledge isneeded with respect to the size and or composition of the contamination,i.e. the type of contamination, present at the predetermined position.Only positions of contamination are relevant. A device capable ofdetecting the presence of contamination, i.e. a device capable ofdetermining contaminated positions on the component to be cleaned, isdescribed in EP 1 093 022 A2, which is incorporated herein by referencein its entirety.

FIG. 2 shows a cleaning system CS in more detail. The cleaning system CSincludes a plasma system for locally providing a plasma. The plasmasystem shown is arranged to provide a shockwave. In use, contaminationoriginating from the resist will be etched away by the plasma. Metal oroxide particles will be blown away by the shockwave. In the drawing, nodifference is made between contaminating resist particles and metaland/or oxide particles. Both types, i.e. contamination in general isreferred to with CP. The plasma system is further arranged to focus alaser beam in the proximity of the predetermined position on a componentto be cleaned. Hence, it is possible to clean very selectively and veryaccurately, enhancing efficiency and reducing cleaning time. In moredetail, the plasma system may include a laser L for providing a laserbeam LB. By use of optical components such as, for example, a mirror Mand a focusing lens FL, the laser beam may be focused in the proximityof the predetermined position PP on the component to be cleaned. Thepredetermined position PP may, for example, be a surface S of aso-called “burl” which is a protrusion of the wafer table WT forsupporting, in use, the wafer. The plasma system may include a beamsplitter BS and a harmonic generator HG, for, respectively, directing alaser beam to the harmonic generator and producing, for example, a UVlaser beam UVLB. It has turned out that with this additional UV laserbeam arriving at the spot FS onto which the laser is focused,advantageous cleaning of the proximity of the predetermined position,and the predetermined position PP itself, may occur in an efficientmanner. The laser may heat the surface to be cleaned, including G-forcesdue to a rapid difference in expansion between the contaminatingparticles and the surface to be cleaned. To further enhance efficiency,it may be necessary to optimize the distance D between thepredetermining position PP and the spot onto which the laser beam LB isfocused. This optimization may be carried out by routine experiments bya skilled person. The distance D may also be pre-set on the basis of forexample required accuracies and a predetermined relation between theaccuracies produced and the distance D. Also an automated optimizationprocedure may be employed for finding a suitable distance D.

The plasma system may be arranged such that the focused laser beam mayselectively scan the surface of the wafer table WT on the basis of thepredetermined positions. With a cleaning system of an apparatusaccording to the invention, cleaning occurs in at least the proximity ofthe predetermined positions, and, thus, in the predetermined positionsitself. For this purpose, optical components may be rotatable and/ortransferable. It is, however, also possible that the whole cleaningsystem is capable of moving along the surface.

FIG. 3 shows an alternative embodiment of a cleaning system as part ofan embodiment of the invention. In this case, a cleaning system includesa heating system for heating the components be cleaned in the proximityof the predetermined position. In particular, the heating system mayinclude a laser that is arranged to provide a beam with a wavelengththat is absorbable by the component to be cleaned. The laser may providea beam that is directed perpendicular to the surface of the component tobe cleaned, or as shown a beam that approaches the surface under anangle. No specific knowledge is required of the type of contaminationpresent at the predetermined position. The laser may further be arrangedto provide a beam with a pulse longer than 0.1 milliseconds, or more inparticular with a pulse longer than 0.5 milliseconds. This embodimentmay include a laser that is arranged to provide a beam with a pulseshorter than 0.15 microseconds or more in particular a pulse shorterthan 0.10 microseconds. This may be the same laser as the laser thatprovides the pulses longer than 0.1 milliseconds or longer than 0.5milliseconds. Two lasers may be employed to meet these requirements. Inuse, the component to be cleaned is heated locally; i.e. in at least theproximity of the predetermined positions. Any wavelength may be chosenas this type of laser beam is simply absorbed by the component to becleaned. There is no need to fine-tune the wavelength such that thecontamination absorbs the laser light. In order to keep costs low, alaser arranged to provide a beam with an infrared wavelength may beused. In particular, when a wavelength of about 1064 nm is used,contamination including both oxide particles and resist particles may beremoved from the component to be cleaned by using this cleaning system.Without intending to be bound by any theory, it is believed that theresist will be heated by conduction and evaporate away from thecomponent to be cleaned, whereas the metal and oxide particles areloosened from the component to be cleaned by high G-forces that resultfrom thermal expansion differences between the wafer table WT and theseparticles. Other advantages of this embodiment are not only related to alow cost laser that may be used for this system, but also to convenientdelivery of the laser light to the required position by means of fiberdelivery.

FIG. 4 shows another cleaning system as part of an embodiment of theinvention. This cleaning system includes a snow system for locallyproviding snow particles JS. In particular, the cleaning system isarranged to provide a jet of snow particles. The snow particles may beformed from CO₂ and/or Ar. Apparatus and methods for providing snowparticles are known from, for example, U.S. Pat. No. 6,099,396 and U.S.Pat. No. 6,066,032, both of which are incorporated herein by reference.For example, liquid CO₂ that is stored under pressure in a container COand expands after being transported through a conduit CD in a nozzle Ncools down and forms soft snow particles, mostly micron-sized. Theremoval of contaminants from the components to be cleaned may occur viadissolving, in the case of organic contaminants (resist), by impulse toremove particles such as metal and/or oxide particles, and by loweringthe adhesive force between the contamination and the components to becleaned, due to rapid cooling of the component to be cleaned and a largethermal “shrinkage” difference between the contaminants and componentsto be cleaned. The snow particles may also clean the resist particlesaway, as a result of an abrasive action. It is possible that thiscleaning system may be combined with, or includes, a heating device forreducing temperature recovery time after cleaning. This embodiment has,in use, shown to be a very efficient cleaning device, low in cost, anddoes not causes damage to the component to be cleaned. The cleaningsystem may operate substantially independent of contaminants and thetype of components to be cleaned.

Although the jet of snow particles should be aligned such that the snowparticles arrive at least in the proximity of the predeterminedposition, very little accuracy is needed. In general, if the spot to becleaned is relatively large, the alignment may be relatively inaccurate.Also, another nozzle may be used for cleaning a relatively large spot.The nozzle may be designed to improve the cleaning efficiency. It is,for example, conceivable that a venturi is employed, as described in“Carbon dioxide snow cleaning—The next generation of cleaning”, R.Sherman, Paul Adams, Precision Cleaning '95 Proceedings, p. 271-300.Also, the shape and size of the actual outlet of the nozzle may beoptimized. It is further possible to apply a single or a doubleexpansion for production of the snow particles. Methods known to adjustthe snow size and velocity during cleaning may also be employed, such asthose described in U.S. Pat. No. 4,806,171, incorporated herein byreference. A position orientation of the nozzle may be optimized tominimize waste of the snow particles. The skilled person may, usingroutine experiments, find an optimal angle at which the nozzle willenclose with the surface of the component to be cleaned, when cleaningis efficient.

It is possible that the snow system may be arranged to apply thesnow-jet in a direction opposite a direction into which the surface tobe cleaned moves due to, for example, a predetermined scanning. Theimpact of the snow particles would then be due to the relative speedincreased, resulting in a higher impulsive force and an improvedcleaning efficiency.

The system may be arranged to automatically adjust the direction and/orangle of a jet of the snow flakes as described in U.S. Pat. No.5,725,154, incorporated herein by reference. Cleaning using the snowsystem may further occur in a stepping mode or a scanning mode.

It is possible that the snow system may be arranged to provide a pulsedjet of snow particles as, for example, described in U.S. Pat. No.5,725,154 incorporated herein by reference. This may be done byswitching a valve, or by, for example, driving the nozzle in anoscillatory manner, leading to a kind of “snow plough” effect asdescribed in U.S. Pat. No. 6,530,823, incorporated herein by reference.A valve may be close to an outlet of the nozzle to minimize start andstop time, and to provide a step response. The valve may be incorporatedin the outlet and as such control a flow of the snow particles. The snowsystem may be arranged such that high velocity gas is injected in thenozzle to accelerate the snow particles leaving the nozzle. This may bewarm gas and/or an ionized gas to respectively reduce the cooling of thecomponent to be cleaned and to reduce static charging of the componentto be cleaned, as described in U.S. Pat. No. 5,725,154. It is alsopossible that the snow system may be arranged to inject low temperaturenitrogen gas on the snow particles as produced during the adiabaticexpansion. The hardness of the snow particles may increase and thatconsequently the abrasive manner of cleaning the component to be cleanedmay be more thorough. It is further possible that the snow system may bearranged to include a control system for measuring and/or controllingthe pressure of gas, for example, CO₂ gas, available to be used forproviding snow particles. The control system may include a pressuregauge and/or, for example, an instrument to establish the mass of acontainer in which the gas is contained. It is possible that thepressure of the gas flowing to the nozzle may be increased. This may becarried out using a compression device or, for example, a devicearranged to heat the container in which the gas is maintained.

A lithographic apparatus that includes a snow system as the cleaningsystem may include a recovery system for reducing down time of thecomponent to be cleaned. The recovery system may be arranged to provideheat to the components which have just been cleaned or are beingcleaned. This may be carried out by providing a heating jet, a heatinglight source, or integration of a heating device in the component to becleaned. It is, for example, conceivable that a heating device isintegrated in the wafer table. It is also possible to reduce therecovery time by minimizing the spot diameter of the jet of the snowparticles and/or to improve the alignment. It is also possible that thesnow system is arranged to intermittently provide a jet of snowparticles as described in U.S. Pat. No. 5,725,154, incorporated hereinby reference. The snow system may be arranged such that when the nozzleis directed from a first position to be cleaned towards a secondposition to be cleaned, no snow particles are provided. When the wholewafer table is regarded as a predetermined position to be cleaned, adedicated nozzle, a ray of nozzles, or a flat wide nozzle may beemployed as, for example, described in U.S. Pat. No. 6,099,396,incorporated herein by reference.

The lithographic apparatus that includes a snow system as the cleaningsystem may be arranged to maintain a relatively low humidity in thesurroundings of a component to be cleaned. It is, however, also possibleto add solvents to the predetermined positions on the components to becleaned as this, in general, improves cleaning efficiency, as describedin U.S. Pat. No. 5,725,154.

To further avoid static charging of the component to be cleaned, thecomponent may be grounded, or, for example, surrounded by ionized air,which can reach the component to be cleaned. It is also possible thatthe tube and/or of the snow system may be grounded, as described in U.S.Pat. No. 6,530,823, incorporated herein by reference. It is possiblethat a lithographic apparatus that includes a snow system as thecleaning system may further include a gas flow control system forproviding a gas flow that aims to remove contaminant particles that havebeen released from the component that has been cleaned.

If the snow particles include carbon dioxide, the snow system mayinclude a carbon dioxide purifier, such as a filter. Such a purifier mayalso be used in snow systems that are arranged to provide snow particlesform a gas different form CO₂.

FIG. 5 shows schematically a cleaning method for in situ removingcontamination from a component in a lithographic apparatus. In a firststep S1, contamination may be detected directly or indirectly. Directlydetecting contamination may occur when dedicated apparatuses may detectcontamination. In that case, immediate tracing of the contamination mayoccur, thereby leading to identifying a number of predeterminedpositions on components to be cleaned. (Step S1, S2 and S3 in FIG. 5.)Such a dedicated apparatus is described in EP 1 093 022, which isincorporated herein by reference. Indirectly detecting contamination mayoccur when required specifications with respect to a device produced bythe lithographic apparatus are not met, or, for example, a bad overlayis observed (bad overlay refers to irreproducibility when a similarpattern is transferred onto a plurality of substrates). In that case, anapparatus as described in EP 1 093 022 A2 may be employed to tracecontamination (S2), which leads to identifying predetermined positionsthat need to be cleaned (S3). Once the positions on which contaminationis present have been determined, i.e. the predetermined positions areknown, cleaning may occur at the predetermined positions (S4), using acleaning system as described above. After cleaning the predeterminedpositions, the lithographic apparatus may be in normal operation. Thiswill eventually lead again to contamination which will be detected andstep S1 may again trigger the occurrence of the cleaning method asoutlined above.

FIG. 6 shows schematically a cleaning method according to an embodimentof the invention. In this case, cleaning a predetermined position isfollowed by normal operation of the lithographic apparatus, which isagain followed by cleaning the predetermined positions. In thisembodiment, the predetermined positions have been predetermined withoutdetecting the presence of contamination on positions. It may, forexample, be the case that alignment markers, and/or locations underand/or around alignment markers form a predetermined position and,respectively, the proximity of a predetermined position. In order tomeet consistently high accuracy requirements, it may be required toperiodically clean such positions without first waiting forcontamination to be detectable. This may not only apply to alignmentmarkers, but also to surfaces that are used for determining a position,such as mirror surfaces. Also, when determining lens qualifications,uncontaminated surfaces are needed. These surfaces may be cleaned by amethod and a cleaning system as described above. In general, theapparatus may be arranged to clean at least one optical component and/orone position sensitive component. The apparatus may further be arrangedsuch that when a position sensitive component, such as the substratetable, is cleaned, both sides of that component can be cleaned.Components to be cleaned may include mirror blocks (either side),optical sensors, and reflective elements of interferometers.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in, for example, atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example, in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example, imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography, atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. An immersion lithographic apparatus comprising: asubstrate table constructed and arranged to support a substrate; aprojection system constructed and arranged to project a patterned beamof radiation onto a target portion of the substrate; a liquid that fillsa space between the projection system and the substrate; and a cleaningsystem for cleaning a predetermined position on a component in theimmersion lithographic apparatus in situ, said cleaning system beingarranged to provide a localized cleaning environment in proximity of thepredetermined position on the component to be cleaned, the systemfurther being arranged to provide the localized cleaning environmentsubstantially independent of a type of contamination present at thepredetermined position, wherein the cleaning system comprises a plasmasystem for locally providing a plasma and an ultraviolet laser beam tothe proximity of the predetermined position such that the ultravioletlaser beam passes through the plasma, wherein the component to becleaned comprises an optical element, and wherein the type ofcontamination is selected from the group consisting of resist particles,metal particles, and oxide particles.
 2. The immersion lithographicapparatus according to claim 1, wherein the plasma system is arranged toprovide a shock wave.
 3. The immersion lithographic apparatus accordingto claim 2, wherein the plasma system is arranged to focus a secondlaser beam in the proximity of the predetermined position on thecomponent to be cleaned.
 4. The immersion lithographic apparatusaccording to claim 3, wherein the plasma system comprises a mirror and afocusing lens configured to focus the second laser beam in the proximityof the predetermined position on the component being cleaned.
 5. Theimmersion lithographic apparatus according to claim 3, wherein theplasma system comprises a laser configured to generate the second laserbeam and a harmonic generator configured to produce the ultravioletlaser beam from the second laser beam.
 6. A cleaning system for cleaninga predetermined position on a component in an immersion lithographicapparatus in situ, the immersion lithographic apparatus comprising asubstrate table constructed and arranged to support a substrate, aprojection system constructed and arranged to project a patterned beamof radiation onto a target portion of the substrate, and a liquid thatfills a space between the projection system and the substrate, thecleaning system being arranged to provide a localized cleaningenvironment in proximity of the predetermined position on the componentthat is substantially independent of a type of contamination, whereinthe cleaning system comprises a plasma system for locally providing aplasma and an ultraviolet laser beam to the proximity of thepredetermined position such that the ultraviolet laser beam passesthrough the plasma, wherein the component comprises an optical element,and wherein the type of contamination is selected from the groupconsisting of resist particles, metal particles, and oxide particles. 7.The cleaning system according to claim 6, wherein the plasma systemcomprises a laser configured to provide a second laser beam and a mirrorand a focusing lens configured to focus the second laser beam in theproximity of the predetermined position on the component to be cleaned.8. The cleaning system according to claim 7, wherein the plasma systemfurther comprises a harmonic generator configured to produce theultraviolet laser beam from the second laser beam.
 9. A cleaning methodfor removing contamination from a predetermined position on a componentin an immersion lithographic apparatus in situ, the immersionlithographic apparatus comprising a substrate table constructed andarranged to support a substrate, a projection system constructed andarranged to project a patterned beam of radiation onto a target portionof the substrate, and a liquid that fills a space between the projectionsystem and the substrate, the method comprising: creating a localizedcleaning environment in proximity of the predetermined position on thecomponent and that is independent of a type of contamination to beremoved, the type of contamination selected from the group consisting ofresist particles, metal particles, and oxide particles; providing aplasma locally to the proximity of the predetermined position; providingan ultraviolet laser beam to the proximity of the predetermined positionand through the plasma; and removing the contamination from thecomponent in situ, wherein the component comprises an optical element.10. The cleaning method according to claim 9, wherein the methodcomprises providing a plasma shock wave.
 11. The cleaning methodaccording to claim 10, wherein the method comprises focusing a laserbeam separate from the ultraviolet laser beam in the proximity of thepredetermined position.
 12. A cleaning method for removing contaminationfrom a predetermined position on a component in an immersionlithographic apparatus in situ, the immersion lithographic apparatuscomprising a substrate table constructed and arranged to support asubstrate, a projection system constructed and arranged to project apatterned beam of radiation onto a target portion of the substrate, anda liquid that fills a space between the projection system and thesubstrate, the method comprising: identifying the predetermined positionto be cleaned on the component, wherein the component comprises anoptical element; cleaning the predetermined position with a cleaningsystem in situ, the cleaning system being arranged to provide alocalized cleaning environment in proximity of the predeterminedposition that is substantially independent of a type of contamination,wherein the type of contamination is selected from the group consistingof resist particles, metal particles, and oxide particles; providing aplasma locally to the proximity of the predetermined position; andproviding an ultraviolet laser beam to the proximity of thepredetermined position and through the plasma.